1. Field of the Invention
The invention relates to a combination fracing and gravel packing assembly for use in a subterranean well.
2. Brief Description of the Prior Art
Subsequent to the drilling of the subterranean oil or gas well, casing is typically set and is perforated in conventional fashion. A fracing fluid may then be injected under pressure through the perforations into the production zone to break up the formation to open up or provide fractures within the zone to permit more efficient production of the well therethrough.
Unconsolidated formations, particularly those containing loose sands and sandstone strata, present problems in well production due to migration of loose sands and degraded sandstone into the well bore after fracturing or as the formation deteriorates under the pressure and flow of fluids therethrough during normal production of the well. Such migration of particles may eventually clog the flow passages in the production system of the well, and can seriously erode the equipment. In some instances, the clogging of the production system may lead to a complete termination of flow, or killing of the well.
One method of controlling sand migration into a well bore consists of placing a pack of gravel on the exterior of a perforated or slotted liner or screen which is positioned across an unconsolidated formation to present a barrier to the migrating sand from that formation while still permitting fluid flow thereacross. The gravel is carried to the formation in the form of a slurry, and the carrier fluid being removed and returned to the surface. The proper size of gravel must be employed to effectively halt sand migration through the pack, the apertures of the liner or screen being gauged so that the gravel will settle out on its exterior, with the slurry fluid carrying the gravel entering the liner or screen from its exterior and being circulated back to the surface.
Prior to effecting the gravel pack, drilling mud and other contaminants may be washed from the well bore, and the formation treated, such as by fracing the well, as above described.
Subsequent to effecting the gravel pack, a reverse circulation technique may be utilized to remove remaining gravel and in slurry from the operating string utilized to conduct the slurry. With such a reverse circulation technique, the direction of circulation is reversed and a clean fluid is pumped down the path previously utilized for returning the slurry fluid, and the remaining gravel and slurry will be forced back up the path originally used to conduct the gravel and slurry down to the well.
Typical gravel packing and fracing devices and methods are shown in the prior art, such as in U.S. Pat. Nos. 3,987,854, entitled "Gravel Packing Apparatus And Method"; 4,606,408, entitled "Method And Apparatus For Gravel Packing A Well"; and 4,627,488, entitled "Isolation Gravel Packer". Such typical and other prior art gravel packing assemblies contain a crossover assembly which permits fluid to be introduced downwardly within the work string or tubular conduit upon which the packer is carried, with the fluid being disposed out of the crossover tool and into a fiuid flow path exterior of the liner assembly and into the annular area in the well between the liner assembly and the casing. The crossover tool also has intermittently spaced fluid return flow paths therein which are offset from the ports or openings within the crossover tool opening into the liner assembly.
It has been observed in such prior art and devices that the turbulence within the downwardly flowing gravel packing or fracing of fluid will result in an actual cutting of the internal wall of the crossover assembly directly into the return fluid flow paths in the device such that planned fluid flow integrity is lost and a circulation path is unintentionally established directing such fluids through the crossover assembly interior and outwardly thereof within the area between the crossover assembly and the liner, around the packer and within the tubing/casing annulus above the packer, to the top of the well. When this problem is encountered, the well must be completely closed in and the work string, packer, and liner assembly with the crossover therein must be completely withdrawn from the well and replaced. This results in considerable down time and extra rig time being utilized.
In particular, many such prior art devices incorporate a hydraulic setting mechanism for the packer which requires that the interior of the crossover assembly above, or upstream, of the crossover ports be isolated from the interior of the assembly and the work string. Such is effected by providing a selectively releasable sliding sleeve defining a valve seat thereon. When it is desired to hydraulically set such packer, a spherical element, such as a ball, is gravitated or pumped through the work string until it sealingly rests upon the valve seat. Thereafter, pressure within the work string is increased and the packer is set. After testing of the integrity of the seated packer, the pressure is increased and a sliding sleeve is caused to be shearably released from the interior of the crossover assembly and it is shifted downwardly and below, or downstream of the crossover ports. Such prior art combination valve head and seat assemblies thus permit the valve head, or ball, to be sealingly held above and on the upper end of the sleeve, whereby the exterior of the ball is exposed to the flow of the fracing and/or gravel packing fluids. The ball, having an outer diameter which is less than the inner diameter of the inner wall of the crossover assembly, will "chatter" or move upon its seat, as the turbulent fluid within the crossover assembly and the work string changes direction to enter into the crossover port. The constant hard contact of the ball against the crossover wall has been found to also result in abrasion of the wall and thereby cutting a hole within such wall to expose the interior of the crossover assembly to the fluid flow return paths within the crossover assembly, thus directly communicating the downwardly flowing fluid with an upwardly exposed fluid flow path at a time when same is not desired.
Turbulent fluid effects upon gravel packing systems during frac/pack operations is discussed to some extent in paper no. 22857 of the Society of Petroleum Engineers entitled "Study Of Effects Upon Gravel-Pack Systems During Frac/Pack Operations", (1991).
The present invention is directed to abating the problems discussed above in the prior art assemblies.